1. Field of the Invention
This invention relates to an information recording medium and also to information recording and erasing methods. In particular, this invention is concerned with an information recording medium having a recording layer with an organic polymer and dye contained therein and also with information recording and erasing methods making use of the information recording medium.
2. Description of the Related Art
Conventionally well-known optical recording media include inorganic optical recording media, such as magneto-optic recording media--in each of which a magnetic film is heated to its Curie point or higher by a laser beam, the direction of magnetization at each heated point is rewritten by the application of a magnetic field and a change in the plane of polarization due the Kerr effect is read--and phase-transforming optical recording media in each of which an optical recording medium is caused to undergo a phase transformation (crystalline phase.fwdarw.amorphous phase) by the heat of a laser beam and the reflectivity of light is read.
Magneto-optic recording media are generally equipped with a thin alloy film made of a rare earth element such as Gd or Tb and a transition metal such as Fe, Ni or Co. These thin alloy films are however prone to oxidation by moisture, oxygen in the atmosphere or the like so that their contact with the external atmosphere must be avoided by a protective film made of silicon oxide, silicon nitride or the like. Magneto-optic recording media therefore have a problem in the long-term stability of records. They are also accompanied by the drawback that they require a complex optical system.
Phase-transforming optical recording media are made of a material containing a chalcogen element led by Te, for example, tellurium oxide, a Te-Ge alloy or a Te-Ge-Sb alloy. However these materials have toxicity, and their formation into films requires a vacuum film-forming process such as a sputtering process thereby making it difficult to provide optical recording media at low cost.
With a view toward overcoming the drawbacks described above, active research is now under way with respect to optical recording media made of an organic material for their reduced toxicity and greater economy.
Optical recording media made of an organic material and reported to date include photon-mode optical recording media making use of photons of light and heat-mode optical recording media utilizing heat.
Among these numerous organic rewritable optical recording media reported so far, there are not many optical recording media whose dynamic optical recording/erasing characteristics have been reported. The capabilities of many such optical recording media is, therefore, unknown in terms of record/erase performance.
As one of a few reports which disclose dynamic optical recording/erasing characteristics with respect to rewritable organic optical recording media, there is a report entitled "Characteristics and Problems of Organic Materials for Optical Recording Media" in The Journal of Society for the Study of Organic Electronics, 41-47 (1991). An optical recording medium disclosed therein is of the photon-mode type, with the claim that high sensitivity and high speed can be expected. It makes use of the photochromism of a heterocyclic fulgide. As a matter of fact, dynamic evaluation of the optical recording medium gave a CN value of 49 dB. It is, however, difficult to apply this optical recording medium to an optical recording system equipped with a semiconductor laser because it requires an Ar laser beam for recording and ultraviolet rays for erasure. It also has problems in stability and repeatability since its read-out stability is low due to the use of a photochromic reaction in both recording and read-out.
As such organic optical recording media, there have also been reported those making use of a photochromic material such as spiropyran [see Japanese Patent Application Laid-Open (Kokai) No. SHO 61-17037)], phase separation of a polymer blend, or a change in the orientation of a liquid crystal polymer. Since these optical recording media are of the photon mode, they have advantages such as high sensitivity and high speed. However, they have problems in stability and repeatability because their read-out stability is low due to the use of a photochromic reaction in both recording and read-out.
As a report disclosing dynamic optical recording/erasing characteristics on an optical recording medium of the heat mode type, on the other hand, there is a report entitled "Optical Storage Technology and Applications" in SPIE, 211-218 (1988). This optical recording medium is provided with a recording layer which has been formed by laminating a polymer layer, which shows viscoelasticity and will hereinafter be called an "expansion layer", and a thermosetting resin layer (hereinafter called a "retention layer") one over the other. Recording on this optical recording medium is performed as will be described next. The retention layer is heated to its glass transition temperature higher, and the expansion layer is also heated to have it undergo thermal expansion so that the retention layer is deformed to form bumps. The retention layer is next cooled to a temperature lower than its glass transition temperature, whereby the bumps are solidified and retained. Erasure of the information so recorded can be effected by heating the retention layer, which include the bumps formed as described above, to a temperature at least equal to its glass transition temperature so that its modulus of elasticity is lowered. By contraction force of the expansion layer which has been in the expanded state, the retention layer is drawn back so that the bumps are flattened. Such formation and flattening of bumps can be reversed and, accordingly, the above optical recording medium is rewritable. This optical recording medium, however, requires the incorporation of different dyes in the expansion layer and retention layer, respectively, and to use two types of laser beams having different wavelengths, one for recording and the other for erasure, because the expansion layer is heated upon recording but, upon erasure, the retention layer alone is heated--i.e., without heating the expansion layer.
Further, Japanese Patent Application Laid-Open (Kokai) No. HEI 2-187390 discloses an optical recording medium with a recording layer, which contains a shape memory resin and a dye, formed directly on a substrate. In this optical recording medium, the recording layer is a single layer and, owing to the use of both heat-induced expansibility and subsequent restorability of the shape memory resin, formation of bumps (recording) and their flattening (erasure) can be conducted by changing the degree of heating. This, therefore, has made it possible to perform both recording and erasure by using a laser beam of a single wavelength. The above patent publication however does not disclose any dynamic optical recording/erasure characteristics. Depending on the optical constants, film thickness, film forming property and the like of the recording material employed in the recording layer, it may not be possible to obtain push-pull signal characteristics sufficient to perform tracking servo. It is difficult to obtain good push-pull signal characteristics, for example, where there is only a small difference in the complex index of refraction between a substrate and a recording layer but there is a large difference in the complex index of refraction between the recording layer and air or the recording layer and a layer laminated thereon (for example, a reflective layer or a protective layer). In addition, poor film-forming property of a recording material leads to problems such as higher noise level and tracking failure due to small roughnesses or the like of the recording material.
As a report disclosing dynamic optical recording/erasure characteristics on an optical recording medium of the combined photon and heat mode, reference is had to Optical Memory Symposium '90, 31-32 (Summer, 1990). The optical recording medium, which uses a liquid crystal polymer having photochromic groups, actually gave a CN value in a dynamic evaluation but it was as low as 27 dB. Moreover, it failed to exhibit erasure characteristics (erasure ratio). This optical recording medium also involves the problem that it can hardly be applied to an optical recording system equipped with a semiconductor laser, because it requires ultraviolet rays for erasure.
As recording medium of the heat mode type that uses heat, there is proposed recording through repetition of pit formation and flattening by the use of a thermoplastic polymer and an organic dye [Japanese Patent Application Laid-Open (Kokai) No. SHO 58-48245]. Because of the reliance on an irreversible change in profile of the recording medium, this recording medium involves the problem that erasure of a record is extremely difficult.